1. Field of the Invention
The present invention relates to the operation of a mill and particularly the exercise of control over a roller mill employed in a fuel feed system of a fuel consuming load such as a steam generator. More specifically, the present invention is directed to a temperature control system for a roller mill which produces, from coal and pressurized gas feeds, a fuel stream comprising coal dust entrained in a carrier gas. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
It is well known in the art to fuel steam generators, vaporizers and other fossile fuel consuming loads by delivering thereto a highly volatile mixture of coal dust entrained by a carrier gas. Such fuel mixtures are produced in roller mills which receive "raw" coal and pressurized air. In the operation of a roller mill, it is known to automatically regulate the amount of coal fed into the mill as a function of the steam generator load. Controls for achieving such coal feed regulation, while at the same time insuring safe mill operation, have typically operated by controlling the volume of carrier gas in accordance with pre-calculated ratios of coal to primary feed air. Safe mill operation requires, in addition to maintaining the appropriate coal/air ratio, that the temperature of the fuel stream exiting the roller mill be maintained within safe limits. This requisite temperature control is accomplished by mixing cold air with the "hot" or primary air delivered to the mill.
A disclosure of a prior art mill having a temperature safety control at the mill outlet is disclosed in the brochure entitled "SUHUSSELMUHLEN/BOWL MILLS" Publication No. 3000-KO 4.88, Page 10.
Prior art mill control apparatus and techniques have not been altogether satisfactory. One particular disadvantage of known roller mill controls, of the type briefly described above, resides in the fact that the air-control valves have a delayed reaction. Restated, employing previous control technology, the mill cannot respond quickly when the rate of change of the steam generator load is high. The conventional method of overcoming this disadvantage is to override the mill control. However, this solution presents the possibility of local overheating in the steam generator or other device to which the pulverized coal/air mixture is being delivered.
It is to be noted that prior art roller mill controls have also contributed to noisy mill operation and have, because of the ability to safely respond to only relatively small load gradients, imposed a limitation upon mill capacity. A further problem related to prior art control techniques has been the discharge of excess coal through pyrite ejection.